The scale on a map is 4cm : 9km. If the distance between two cities is 81km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 4cm on the map corresponds to an actual distance of 9km. An actual distance of 81km is the same as 9 $\cdot$ 9km. The distance between the two cities on the map is 9 $\cdot$ 4cm, or 36cm.